1. Field of the Invention
The present invention relates to a piezo valve with a piezo-electric bending element, this having a control area that can be moved against an elastically applied opposing force as a function of an applied control voltage.
2. The Prior Art
Valves of this kind which, in keeping with their most frequent application, are frequently referred to as electro-pneumatic converters, are already known. These incorporate a piezo-electric, mostly a piezo-ceramic bending element as a regulating element, and this is fixed at one end. When it is in the switch position in which no voltage is applied, the moveable control area of the bending element that is opposite the end that is clamped is pre-tensioned and lies against an air-inlet valve such that this valve is closed so as to be pressure-sealed. When a control voltage is applied, the bending element is then raised from the air inlet valve seat, against the pre-tensioned counterforce, which means that this valve is then opened to permit the flow of medium. Also known are arrangements in which, according to the same principle, there is a changeover between an inlet and an outlet valve seat, so that a selectable change in the flow of the medium can be effected by the position of the control area.
Because of the properties of piezo-electric materials, the bending elements in this type of valve entail the disadvantage that if they are acted upon mechanically and/or electrically over a longer period of time, their rest position changes. The corresponding property is also referred to as relaxation, and a distinction is drawn between two basic types. The change in shape that takes place under mechanical loading over a longer period of time is referred to as mechanical relaxation, and this is oriented against the direction of the pre-tensioning force. The change in shape that can be identified when an electrical control voltage is applied over a longer period of time, and which is perceived as an increase in the bending, is referred to as electrical relaxation, and this acts in the direction of the electrical bending. It is known that both the mechanical and the electrical relaxation can only be reversed in part and, taken together, they change the adjusted position of the bending element to an unacceptable extent that can, for example, lead to the fact that, after the bending element has been acted upon for a longer time by the voltage, the control area of this will not return to the other switched position, even after the control voltage is switched off and this, in turn, can mean that the valve seat that is associated with the resting position can no longer be closed.
It is an object of the present invention to so improve a piezo valve of the type described in the introduction hereto, that the disadvantages addressed above, which are a consequence of electrical and mechanical relaxation, are avoided and that, in particular, it can be ensured that the bending element functions reliably even after longer-lasting switching cycles.